A comparison of awake and sleeping cortical states by analysis of the somatosensory-evoked response of postcentral area 1 in rhesus monkey

The response of primary somatosensory cortex (SI) to cutaneous stimulation of the hand was studied in one unanesthetized monkey to examine more closely the previously reported dependence of a behaviorally relevant late component of the evoked potential, N1, upon two behavioral states of arousal, slow-wave sleep (SWS) and inactive wakefulness. Simultaneous recordings of the somatosensory-evoked potentials (SEP), multiple unit activities (MUA) and current source-densities (CSD) at 12 depths through area 1 were collected during both states and analyzed to identify the cortical events that generate the SEP. As previously reported, the N1 component of the SEP was diminished during sleep. Although the earlier components of the SEP were evoked during both states, the excitatory events that characterize the awake N1 were replaced during SWS by an interval of strong inhibition which was terminated by a burst of cortical MUA. CSD analysis of the SWS inhibitory interval revealed large current sources through layer III above the depth of maximal MUA inhibition indicating that an outward conductance became activated on the apical trunks of pyramidal neurons during SWS. These data support the proposed involvement of events underlying N1 in the coding of touch experience.